Manchester United can expect another thorough examination of their European credentials when they face FC Porto in Portugal on Wednesday night.
Any suggestion of a quarter-final walkover for the reigning champions and betting favourites was dispelled within minutes of last week's first leg starting at Old Trafford.
United were given a rough ride by the Portuguese champions last Tuesday, finding themselves behind in the fourth minute to a Cristian Rodriguez stunner.
Sir Alex Ferguson's side rode an early storm from the visitors before hitting back through Wayne Rooney and Carlos Tevez to lead 2-1 with five minutes remaining.
But Mariano Gonzalez's 89th-minute strike not only restored parity, it also secured a potentially priceless second away goal for coach Jesualdo Ferreira's plucky side.
United must now become the first English side to win a Champions League tie at Porto's intimidating Estadio do Dragao stadium if they are keep their dream of back-to-back European crowns alive.
Ferguson will also be acutely aware that United have lost their last four knockout ties in the Champions League when they have played the second leg away from home.
But the European champions are likely to have Rio Ferdinand available again following a back problem to marshal a defence which has looked increasingly suspect in recent weeks.
Dimitar Berbatov is also fit and could start in attack and the languid Bulgarian could be vital to his side's chances with goals being the name of the game.
The night's other quarter-final tie is also balanced on a knife-edge as Arsenal and Villarreal renew rivalries with the score level at 1-1 after the first leg in Spain.
Emmanuel Adebayor's stunning overhead kick pulled his side level at El Madrigal last week and the away goal has put the Gunners in the driving seat in the tie.
Arsenal's chances have also been boosted by the absence of Villarreal's influential captain Marcos Senna.
The Spain international had fired his side into an early lead last week with a 30-yard thunderbolt, but he limped out of the weekend defeat at the hands of Malaga with a hamstring injury which will sideline him on Wednesday.
Arsenal have injury problems of their own as key defender William Gallas has been ruled out for the season with the knee ligament damage he picked up in the first leg.
Understudy Johan Djourou limped out of the 4-1 weekend victory at Wigan Athletic, while goalkeeper Manuel Almunia will also be a spectator after suffering a recurrence of an ankle problem in Spain.
But Gunners boss Arsene Wenger will be buoyed by the fact that his side are unbeaten in 23 home matches in the Champions League - a run stretching back to 2004.